1. Field of the Invention
The present invention relates to an acceleration sensor including a piezoresistive element.
2. Description of the Related Art
An example of an acceleration sensor including a piezoresistive element is an acceleration sensor that detects an acceleration in the direction of the thickness of the acceleration sensor and an acceleration in the direction of a plane of the acceleration sensor disclosed in International Publication No. WO 2005/062060 or Japanese Unexamined Patent Application Publication No. 2004-340858.
FIG. 9A is a plan view of an acceleration sensor 1P based on International Publication No. WO 2005/062060. The acceleration sensor 1P includes a support frame 2P, a weight 3P, four flexible arms (beams) 41P, 42P, 43P, and 44P, an X-axis detection piezoresistor 51P, a Y-axis detection piezoresistor 52P, and a Z-axis detection piezoresistor 53P. The support frame 2P has a rectangular frame shape. The weight 3P is arranged inside the support frame 2P. The four flexible arms 41P, 42P, 43P, and 44P are arranged in a cross shape around the weight 3P, and each of them is connected between the weight 3P and the center of the corresponding side of the support frame 2P. The X-axis detection piezoresistor 51P is disposed on the flexible arms 42P and 44P. The Y-axis detection piezoresistor 52P is disposed on the flexible arms 41P and 43P. The Z-axis detection piezoresistor 53P is disposed on the flexible arms 42P and 44P.
FIG. 9B is a plan view of an acceleration sensor 1Q based on Japanese Unexamined Patent Application Publication No. 2004-340858. The acceleration sensor 1Q includes a semiconductor substrate 2Q, a weight 3Q, a beam portion 4Q, a Z-axis detection piezoresistor 51Q, and an X-axis detection piezoresistor 52Q. The semiconductor substrate 2Q has a rectangular frame shape. The weight 3Q is arranged inside the frame of the semiconductor substrate 2Q. The beam portion 4Q extends along the Y axis and is connected to the semiconductor substrate 2Q and the weight 3Q. The beam portion 4Q includes a surface 41Q orthogonal to the Z axis and parallel to the X axis and Y axis and an oblique surface 42Q that is not orthogonal to the Z axis or X axis and that is parallel to the Y axis. The X-axis detection piezoresistor 52Q is disposed on the oblique surface 42Q and detects an acceleration in a direction along the X axis. The Z axis detection piezoresistor 51Q is disposed on the surface 41Q and detects an acceleration in a direction along the Z axis.
The acceleration sensor disclosed in Japanese Unexamined Patent Application Publication No. 2004-340858 needs the oblique surface 42Q in the beam portion and the X-axis detection piezoresistor 52Q on the oblique surface 42Q. In order to dispose the oblique surface 42Q in the beam portion and the X-axis detection piezoresistor 52Q on the oblique surface 42Q, sophisticated process techniques are required, and this is a factor for unstable qualities of acceleration sensors.
If the plurality of flexible beams fixed on different positions in the acceleration sensor disclosed in International Publication No. WO 2005/062060 are bent in response to the effects of external stresses, although it is in a state where a desired acceleration is not exerted thereon, an unnecessary detection signal may be output. Additionally, both of the acceleration sensors disclosed in International Publication No. WO 2005/062060 or Japanese Unexamined Patent Application Publication No. 2004-340858 have a problem in that a widened detection range causes a reduction in detection sensitivity.